The conversion of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) to mevalonate is an early and rate-limiting step in the cholesterol biosynthetic pathway. This step is catalyzed by the enzyme HMG-CoA reductase. Statins inhibit HMG-CoA reductase from catalyzing this conversion. As such, statins are collectively potent lipid lowering agents.
Atorvastatin and pharmaceutically acceptable salts thereof are selective, competitive inhibitors of HMG-CoA reductase. Atorvastatin calcium is currently sold as LIPITOR® having the chemical name [R—(R*,R*)]-2-(4-fluorophenyl)-β,δ-dihydroxy-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-1H- pyrrole-1-heptanoic acid calcium salt (2:1) trihydrate and the formula
As such, atorvastatin calcium is a potent lipid-lowering compound and is thus useful as a hypolipidemic and/or hypocholesterolemic agent. Atorvastatin calcium is also useful in the treatment of osteoporosis, benign prostatic hyperplasia (BPH) and Alzheimer's disease.
A number of patents and published International Patent Applications have issued describing atorvastatin, formulations of atorvastatin, as well as processes and key intermediates for preparing atorvastatin. These include: U.S. Pat. Nos. 4,681,893; 5,273,995; 5,003,080; 5,097,045; 5,103,024; 5,124,482; 5,149,837; 5,155,251; 5,216,174; 5,245,047; 5,248,793; 5,280,126; 5,397,792; 5,342,952; 5,298,627; 5,446,054; 5,470,981; 5,489,690; 5,489,691; 5,510,488; 5,686,104; 5,969,156; 5,998,633; 6,087,511; 6,121,461; 6,126,971; 6,433,213; 6,476,235; 6,605,759; WO 01/36384; WO 02/41834; WO 02/43667; WO 02/43732; WO 02/051804; WO 02/057228; WO 02/057229; WO 02/057274; WO 02/059087; WO 02/083637; WO 02/083638; WO 03/011826; WO 03/050085; WO 03/07072; and WO 04/022053.
It has been described that the amorphous forms of a number of drugs exhibit different dissolution characteristics and in some cases different bioavailability patterns compared to the crystalline form (Konno T., Chem. Pharm. Bull., 1990; 38:2003-2007). For some therapeutic indications, one bioavailability pattern may be favored over another.
Variations in dissolution rates can make it advantageous to produce atorvastatin formulations in either crystalline or amorphous forms. For example, for some potential uses of atorvastatin (e.g., acute treatment of patients having strokes as described in Takemoto, M.; Node, K.; Nakagami, H.; Liao, Y.; Grimm, M.; Takemoto, Y.; Kitakaze, M.; Liao, J. K., Journal of Clinical Investigation, 2001; 108(10): 1429-1437), a rapid onset of activity may be highly beneficial in improving the efficacy of atorvastatin.
The preparation of amorphous atorvastatin has been previously described. For example, Lin et al., U.S. Pat. No. 6,087,511, describe forming amorphous atorvastatin from crystalline atorvastatin. To form amorphous atorvastatin, Lin et al. describe that crystalline atorvastatin is dissolved in a non-hydroxylic solvent such as tetrahydrofuran. The non-hydroxylic solvent is removed to produce a brittle foam that is broken up by mechanical agitation to afford amorphous atorvastatin.
WO 00/71116 also describes forming amorphous atorvastatin using a non-hydroxylic solvent.
WO 01/28999 describes a process for forming amorphous atorvastatin calcium by recrystallization of crude atorvastatin from an organic solvent which comprises dissolving crude amorphous atorvastatin calcium in a lower alkanol containing 2-4 carbon atoms or a mixture of such alkanols under heating and isolating the amorphous atorvastatin calcium precipitated after cooling.
WO 01/42209 describes preparing amorphous atorvastatin by precipitating the atorvastatin using a solvent in which atorvastatin is insoluble or very slightly soluble, from a solution of atorvastatin which is provided with a solvent in which atorvastatin is freely soluble. Preferred solvents in which atorvastatin is freely soluble include low molecular weight alcohols, e.g. methanol and ethanol.
WO 03/078379 describes forming amorphous atorvastatin by dissolving atorvastatin in a hydroxylic solvent and removing the solvent by either freeze-drying or spray drying.
US Published Patent Application 2004/0024046 A1 describes a process for forming amorphous atorvastatin by precipitating atorvastatin from a solution with a solvent in which atorvastatin is insoluble or very slightly soluble.
The use of amorphous pharmaceutical substances, such as amorphous atorvastatin calcium, can be beneficial because such disordered materials usually have higher solubility and bioavailability. However, an unsatisfactory characteristic shared by amorphous drug substances is that they usually have lower physical and chemical stabilities, thus reducing their shelf life over crystalline drug substances. Thus, amorphous atorvastatin is susceptible to degradation upon storage. Once degraded, the drug material may not be acceptable for some medical uses. As a result, there is a need to develop methods for stabilizing amorphous atorvastatin.